1. Field of the Invention
The present invention relates to a glass substrate and a capacitance-type pressure sensor having, as a pressure sensing member, a silicon diaphragm on a glass substrate.
2. Description of the Related Art
This type of pressure sensor includes a differential pressure sensor for measuring relative pressure and an absolute pressure sensor for measuring absolute pressure.
FIG. 10 is a cross-sectional view schematically showing the structure of a conventional capacitance-type pressure sensor. A capacitance-type pressure sensor 1 shown in FIG. 10 is formed by bonding a glass substrate 4 to a silicon substrate 3 having a pressure sensing diaphragm 2, which is a movable electrode to which pressure to be measured is applied. An electrode 5 is provided on a surface of the silicon substrate 3 facing the glass substrate 4. The pressure sensing diaphragm 2 is separated from the glass substrate 4 at a predetermined gap, and a space 6 is formed therebetween. A fixed electrode 7 is formed in the space 6 on the glass substrate, and a connecting electrode 8 is formed on a bottom surface and a side surface of a through hole 4a passing through the glass substrate 4 so as to be electrically connected to the fixed electrode 7 (for example, see Japanese Patent No. 2772111).
The conventional capacitance-type pressure sensor shown in FIG. 10 is manufactured as follows. First, the through hole 4a is formed in the glass substrate 4 by a sand blast process, and the glass substrate having the through hole 4a therein is bonded to the silicon substrate. Then, silicon is removed with one covering the through hole 4a left. Subsequently, the fixed electrode 7 and the connecting electrode 8 are formed so as to be electrically connected to the remaining silicon, and then the silicon substrate 3 having the pressure sensing diaphragm 2 and the electrode 5 facing the glass substrate 4 is bonded to the glass substrate 4 such that the space 6 is formed therebetween.
However, in the conventional capacitance-type pressure sensor, as described above, the through hole 4a is formed by the sand blast process, and the connecting electrode 8 is formed on the side surface of the through hole 4a. In general, since the sand blast process causes the processed surface to be excessively rough, it is difficult to mount a connecting electrode 8a on the processed surface with high precision, which causes the connecting electrode 8a to be broken. Further, in this structure, since the processed surface becomes rough, a poor connection of the connecting electrode occurs, which results in a lower degree of airtightness. When the degree of airtightness is deteriorated, the pressure sensing diaphragm is not normally operated, which makes it difficult to accurately measure a pressure variation.